Sustained
stress calculation is specific in B 31.1 but undefined for B 31.3.

B 31.1
intensifies torsion but B31.3 does not intensify it.

Question No. 19

How to calculate the basic allowable stress
for a material?

Answer:
The basic allowable stress is defined in respective code. For example as
per B 31.3 the basic allowable stress for a material is the minimum of the
following:

1/3rd of tensile
strength at design temperature.

2/3rd of
yield strength at design temperature.

100% of
average stress for a creep rate of 0.01% per 1000 hours.

67% of
average stress for rupture at the end of 100000 hours.

80% of
minimum stress for ruptures at the end of 100000 hours.

For
austenitic stainless steel or nickel alloys the lower of yield strength and 90%
of yield strength at temperature.

For
structural grade materials 0.92 times of the lowest value of point (a) to (f)

Question No. 20

What is the main difference between
Constant and Variable Spring Hanger? When to use these hangers?

Answer:
In Constant Spring hanger the load remains constant throughout its travel
range. But In variable spring hanger the load varies with displacement.

Spring
hangers are used when thermal displacements are upwards and piping system is
lifted off from the support position. Variable spring hanger is preferable as
this is less costly.

Constant
springs are used:

When thermal displacement exceeds 50 mm

When variability exceeds 25%

Sometimes when piping is connected to strain
sensitive equipment like steam turbines, centrifugal compressors etc and it
becomes very difficult to qualify nozzle loads by variable spring hangers,
constant spring hangers can be used.

Question No. 21

What do you mean by variability? What is
the industry approved limit for variability?

Can you select a proper spring hanger if
you do not make it program defined in your software?

What is the procedure?

Answer:
In your system first decide the location where you want to install the
spring. Then remove all nearby supports which are not taking load in thermal
operating case. Now run the program and the sustained load on that support node
is your hot load. The thermal movement in that location is your thermal
movement for your spring. Now assume variability for your spring. So calculate